An approach to the further understanding of the pathophysiology and etiology of diarrheal states in adults and children is proposed. Three inter-related areas of study are being undertaken: (1) validation of a pathophysiologic basis for classifying diarrheal states, (2) determination of a common mechanism(s) for the secretory diarrheas and (3) identification of specific etiologic agent(s), which may be associated with secretory diarrhea. In order to accomplish these objectives patients with diarrhea are studied over a three day period during the acute and convalescent periods of illness. The measurement of 24 hour stool volume, osmolality and electrolytes under fed and fasting conditions is used to differentiate patients into osmotic or secretory diarrhea groups. Multiple intestinal biopsies are obtained for biochemical, histologic, bacterial and viral studies. These data will be analyzed by comparing the type of diarrhea (secretory or osmotic) with the biochemical data (pathophysiology of diarrhea). The validity of the presumed correlation between the type of diarrhea, pathophysiology and etiology will be analyzed statistically utilizing the large number of patients studied.